Preparation of cellulose xanthate solutions



I Patented Mar. 29, 1938 UNITED STATES PATENT OFFICE PREPARATION OF CELLULOSE XANTHATE SOLUTIONS George ALRichter, Berlin, N. 11., assignor to Brown Company, Berlin, N. H., a corporatio of Maine No Drawing. Application March I, 1937, Serial No. 128,492

In preparing cellulose xanthate solutions or viscose for conversion into artificial silk, films,

appropriate alkalinity. The plant space, equipment, and labor costs for performing the various steps of such a process are quite expensive; and,

moreover, it takes days before the wood pulp used as raw material is transformed into a cellulose xanthate solution or viscose of the desired qualy.

I have conducted extensive research'on the preparation of cellulose xanthate solutions with a view toward accomplishing in a single operation or step the transformation of cellulosefiber into a cellulose xanthate solution or viscose of quality suitable for such purposes as manufacturing atificial silk, films, etc., and thus reducing materially the plant space, equipment, labor costs, and time heretoforerequired in so doing. As can readily be appreciated, the success of such research depended upon the realization of cellulose xanthate solution or viscose substantially free from residual or unxanthated cellulose and capable of yielding artificial silk, films, etc., of substantially as good physical and chemical properties as the same ultimate products derived from cellulose xanthate solution as heretofore prepared.

As disclosed in my application Serial No. 37,- 043, filed August 20, 1935, I have found that one factor contributing very importantly to the successof a one-step xanthating process is the viscosity of the cellulose fiber used as raw material and that it is only when the fiber is of a solution viscosity less than 1 and preferably from 0.2 to 0.4 that it reacts smoothly and substantially completely in a single mixing ,operation with the caustic soda solution and carbon bisulphide calculated to yield viscose of the desired cellulose content and causticity, for instance, a cellulose and caustic soda content of about 6% to 10% each. I have now found that while solution viscosity of the fiber is a very important criterion of the suitability of the cellulose fiber for a onestep xanthating operation, there is another factor that controls the suitability of the fiber for such purpose, namely, thebeta cellulose content of the fiber and that unless the beta cellulose content of the fiber falls within a certain range, the fiber may not be satisfactorily xanthatable into 'a-viscose-syrup by the one-step xanthating operation even when it is possessed of the requisite low solution viscosity. To be sure, the fiber should have an appropriately high alpha cellulose content in order to yield viscose of suitable quality, particularly for such purposes as rayon and film manufacture, but it came as a. distinct surprise to me that the beta cellulose content of the fiber also importantly influences the one-step xanthating reaction and further that in order to arrive at consistently satisfactory viscose by the one-step xanthating operation the beta cellulose content' of the fiber must fall within a certain range higher than that of the pulps heretofore used as raw material for-viscose-making. In accordance with the present invention, therefore, I use as raw material for a one-step xanthating operation wood pulp or other cellulose fiber which not only possesses an appropriate alpha cellulose content and appropriately low solution viscosity but whose beta cellulose content falls outside the beta cellulose content range of the wood pulps generallyused heretofore in the usual process of viscose-making.

The industry devoted to usual viscose-rayon manufacture has heretofore generally employed as raw material bleached sulphite wood pulp having the following characteristics:

Alpha cellulose content 88 to Beta cellulose content 3 to 5% Gamma cellulose content 4 to 8% Viscosity 3.0 to 6.0-

the usual viscose-rayon process, the pulpentering into the one-step xanthating process hereof is characterized by lower gamma cellulose content but higher beta cellulose content. In other words, the present invention can be regarded as involving the use as raw material of a pulp wherein the gamma cellulose content of the usual pulp is, replaced to a large extent by beta cellulose which, when kept within a certain range, does not detract significantly from the strength and other qualities of the rayon producible from the pulp while at the same time promoting the desired one-step xanthating reaction on the pulp.

The cellulose fiber for the purposes hereof may be produced from cellulose fibers of various origins, for instance, bleached sulphite. bleached kraft, orother bleached chemical wood pulps,

particular amount of sodium hypochlorite or other oxidant employed depends in large measure upon the solution viscosity of the starting pulp; and in any event sufllcient oxidant is used to cause a reduction of the solution viscosity of the pulp to a, value below 1, preferably distinctly below 1, as hereinafter noted. The treatment of the pulp with the solution may be carried out at a temperature of about 35 to 45 C. for a period of about two to four hours, whereupon the pulp may be washed. A typical washed product resulting from the foregoing processing has the following characteristics.

Alpha cellulose content -1 90% Beta cellulose content 8% Gamma cellulose content 2% Viscosity 0.35

Such a pulp product is eminently suitable for the one-step xanthating process hereof.

It is possible to vary the conditions under which bleached sulphite or other chemical wood pulp is processed to produce the pulp product required for the purposes hereof. Thus, the caustic soda solution may be of 6% to 11% or greater strength and, as already indicated, may contain various amounts of oxidant, depending upon the viscosity of the original pulp. Such a solution may be applied to the pulp at temperatures ranging from 30'to 60 'C., within which temperature range it is possible to increase the beta cellulose content of the pulp so that its percentage falls within the range desired in the finished pulp while decreasing the gamma cellulose content of the pulp .to the appropriately low value and/or keeping it at such value. In any particular case, the various conditions under which the pulp is processed are so correlated as to result in a finished pulp product having the following characteristics:

Alpha cellulose content 87 to 93% Beta cellulose content 6 to 12% Gamma cellulose content 1 to 3% Viscosity preferably distinctly less than 1 The alpha and beta cellulose contents of such a finished pulp product are very significant. Thus,

when the alpha cellulose content of a pulp is appreciably greater than 93%, it is generally difficult to convert such pulp by a one-step xanthating reaction into cellulose xanthate solution of the desired filterability and freedom from unxanthated or residual cellulose or gelatinous cellulose particles. It is evidently the case that the beta cellulose content of the finished pulp product hereof is readily and completely xanthatable and induces ready and complete xanthation of the alpha cellulose content of the pulp in a onestep xanthating reaction. Although the role played in the xanthating reaction by the beta cellulose of such a pulp is not fully understood,

it may be the case that the xanthating reagents, as they react quickly on the substantial amount of beta cellulose occurring in and throughout the fiber wall or structure, are brought immediately into contact with more fiber surface throughout the fiber wall and, because of the resulting intimacy or large surface of contact with the alpha cellulose content of the fiber, react on the alpha cellulose more vigorously than is otherwise the case. As already indicated, the xanthation of cellulose pulp containing gamma cellulose means that the gamma cellulose appears as undesirable degraded cellulose in the viscose; and since in the one-step xanthating process hereof the gamma cellulose content of pulp is not partially extracted with caustic soda solution as in the usual viscose-rayon process but is completely xanthated, the gamma cellulose content of the pulp hereof must be kept at a minimum in order to enable the preparation of satisfactory viscose therefrom. Indeed, if the alpha cellulose content of the pulp hereinbefore described is permitted to fall to a value much below 87% with a corresponding rise in its gamma cellulose content, the pulp, although readily xanthatable to completion in a one-step xanthating reaction, is unsuitable for viscose-making by reason of the fact that the rayon producible therefrom is lacking in requisite strength and other qualities.

The one-step xanthating reaction on the pulp used herein as raw material for viscose-making may be conducted as disclosed in my application Serial No. 37,043, filed August 30, 1935, according to which the pulp hereof is admixed with water, caustic soda, and liquid carbon bisulphide in pro portions calculated to lead directly to a cellulose xanthate solution of about 6% to 10% cellulose and caustic soda content each. In some instances, the pulp may advantageously be sheeted on a papermaking or board-forming machine and the sheets or boards then reduced to small pieces or chips for the one-step xanthating operation, as disclosed in my application Serial No. 58,539, filed January 10, 1936. According to such latter application, the pieces or chips of interfelted pulp fibers are first soaked with caustic soda solution of mercerizing strength so as to soften and swell them, whereupon the pieces or chips may be disintegrated in the presence of diluting water into a fiber suspension substantially devoid of fiber aggregates and the suspension, whose causticity and fiber content has preferably been controlled during initial soaking and dilution 'to yield a finished xanthate solution of a cellulose and caustic soda content each falling within the range of 6% to 10%, is then treated with carbon bisulphide and mixed therewith until the ingredients have reacted substantially to completion to yield the desired cellulose xanthate solution.

A procedure embodying the present invention and involving the use of pulp in chip form, the xanthation of the defib erized chips in the presence of caustic soda and water in amount corresponding to -a caustic soda solution of greater than 10% strength, and the dilution of the resulting viscose or cellulose xanthate solution with water to the desired final cellulose and caustic soda contents may be carried out substantially as follows. The pulp chips, which may consist of pulp having an alpha cellulose content of..90%, a beta cellulose content of 8%, a gamma cellulose content of 2%, and a solution viscosity of, say, 0.3, may be wetted with 16% caustic soda solution at 20 C. in sufllcient volume to provide wetted chips wherein the dry cellulose to caustic soda ratio is 7.6. The volume of 16% causticsoda v the type disclosed in application SerialNo. 53,689,

filed December 10, 1935, so as to produce the thick or salve-like mass, which defiberizing or disintegrating operation may require about fifteen min-- utes in such apparatus. To the resulting thick mass of cellulose fiber and caustic soda solution may then be added liquid-carbon bisulphide in the amount of about 33% and pine oil in the amount of about 0.01%, based on the weight of dry cellulose. The pine oil tends to react with the caustic soda to form a soapy emulsifying agent which promotes microscopically fineparticle size emulsification of the carbon bisulphide in the caustic soda solution and more rapid and complete xanthation of the cellulose, as disclosed more particularly in application Serial No. 63,876,

filed February 14, 1936. Upon operating the mixing blades of the apparatus and tumbling the apparatus as described in application Serial No. 53,689 for a further period of about three hours with the charge of the reacting ingredients at about 20 0., 'it is found at the end of such period that the ingredients have yielded a thick viscose or cellulose xanthate solution substantially free from residual fiber or gelatinous particles and containing about 14% cellulose, based on the weight of viscose. Water may then be added to the apparatus with its mixing blades in operation in amount to dilute the viscose to the desired final cellulose content, say, 7.4%, based on the weight of the finished viscose, at which cellulose content, the resulting viscose has a caustic soda content of 6.7%, based on the weight of viscose. Such a finished viscose is typical in its cellulose and caustic" soda contents of those desired for rayon manufacture. It is to be understood that xanthation may be effected hereby in the presence of water and caustic soda in amounts to correspond to caustic soda solution of greater than 16% strength, say, 18% to 20%, or even greater strength, whereupon the resulting viscose or cellulose xanthation solution may be diluted to the desired final cellulose content and causticity. However, it is usually desirable to use herein, as already indicated, caustic soda solution of such strength and in' such volume for the impregnation of the cellulose fiberor chips as,

to make possible easy and uniform impregnation of the fiber with the caustic soda and the direct xanthation of the resulting mixture, that is, with.- out need of excess caustic soda solution and hence without need of removing excess-solution from the mass prior to xanthation.

The process hereofadmiis of considerable varadded. Thus, while a finished viscose syrup of a causticity and cellulose content of about 7% each has been cited as being conventional and desirable for artificial silk and film manufacture, yet it is possible to produce finished viscose solutions by the process hereof of much lower or higher causticity and/or, cellulose content. Again, the xanthating reaction has been described as preferably being conducted at about "15 to 20 C. (i. e. ordinary temperatures) so as to dispense with the need and expense-of any great refrigeration or cooling, but itis possible and, in some cases may be preferable, to carry out the xanthating reaction at much lower than room temperatures, say, temperatures ranging from slightly above the freezing point to room temperature. In those instances when the xanthating reaction is performed at appreciably lower than room temperature, it has been observed that viscose solutions'of excellent clarity are realized, although. the mixing of the ingredients requires more power by reason of the markedly higher viscosity of the reacting mixture at such temperatures. The initial soaking or treatment of the pulp with caustic soda solution to form the thick or salve-like suspension subjected to xanthation may also be performed at distinctly sub-room temperatures down to the freezing point and with caustic soda solutions of varying causticity, depending-upon the causticity to be realized in the finished xanthate solution. In other words, the causticity of the solution initially employed should be sufiiciently great so as to enable the realization of a xanthate solution which directly after the xanthating eaction or upon subsequent dilution with water as the desired causticity in combination with the desired cellulose content. Itv is thus seen that the present invention although tied down to the use of a pulp of particular combined chemical characteristics in a onestep xanthating process, nevertheless lends itself to considerable variation in so far as concerns the various conditions such as temperature, causticity, cellulose content, carbon 'bisulphide usage, etc. under which xanthation is eflecteda The pulp or'fiber thus used may be of various origins and in various physical conditions. Thus, the pulp may be derived from hardwoods, such as birch or maple, or from softwoods, such as spruce and fir. Again, the pulp may be used with its fibers in substantially unreduced or uncut condition or it may be broken down into fiber fragments or even powdered cellulose by suitable mechanical treatment prior to subjecting it to the one-step xanthating process hereof.

I wish again to stress the fact that the pulp used for the purposes hereof should have an alpha cellulose content of upwards of about 87%, for, when its alpha cellulose content is very much below such value, viscose prepared therefrom by-the one-step xanthating process does not yield rayon'or films of satisfactory quality. That the relatively high beta cellulose content -in-the pulp used herein must be associated with an alpha cellulose content in the pulp of not less than 87% may better be understood from the fact that should cotton or wood pulp be subjected to an acid-hydrolyzing treatment serving to reduce alpha cellulose content in the fiber Y to, say, 75% or 70% and to raise beta cellulose content to upwards of, say, 10%, the resulting cellulose fiber, although. lending itself to quick and complete conversion into viscose by the onestep xanthating reaction, yields as a result of such reaction a viscose'quite unsatisfactory for rayon and film manufacture. The point is, as already indicated, that whereas in the usual viscose-making process a substantial proportion of the non-alpha cellulose components in the pulp are removed by extraction in the excessstrong caustic soda solution used for preparing soda cellulose, such components remain in the mixture of reacting ingredients in a one-step xanthating process. Accordingly, when hydrolyzed fiber of the composition just cited serves as raw material in a one-step xanthating process, the resulting viscose is so greatly contaminated by degraded celluloses that the rayon, films, or other regenerated cellulose products manufactured therefrom are of unsatisfactory physical and chemical qualities. I

Another very important property of the pulp hereinbefore described used for the purposes hereof is the excellent filterability of viscose prepared therefrom by the one-step xanthating process hereof. I have found that unless viscose syrup has a filterability of at least 300 cubic centimeters, as determined by a test hereinafter given, it is not suitable for such practical purposes as the manufacture of rayon. The pulp used for the purpose hereof answers this requirement admirably, as viscose syrup prepared therefrom by the one-step xanthating process hereof has a filterability of at least 500 'cubic centimeters.

I shall now give the method used herein for determining the filterability of viscose solution.

V scose samples Filter chamber This chamber is a pipe having an internal crosssection of 7.0 square inches and an overall length between its two flanged ends of 48 inches. It is mounted in a vertical position. It is covered with a disc on its top, with a perforated rubber gasket between this disc and the top flanged end. This disc is bolted on after an orifice plate, the filter pad, a support, and a discharge pipe have been attached to the lower flanged end, and the viscose sample added to the chamber. The viscose sample may be insufllcient to fill the chamber. If it does fill the chamber, its weight is a maximum of only two pounds to the square inch at the filter pad. The top disc carries a pipe connection to a supply of compressed air through an air separator and reducing valve.v This valve is set to give an air pressure of 70 pounds on the sample of viscose. The air to the reducing valve is always at a pressure greater than 70 pounds.

The chamber is surrounded by a second pipe to give a V2" water jacket. This jacket is welded to the inner faces of the two flanged ends of the unit. A stream of water is circulated up through the jacket from a header supplied by both cold and warm water services. A thermometer pocket is built into this header. The water to the jacket is held at 20 C. by manual control of the water services as read by a thermometer in the pocket. The discharged water is wasted into a drain.

Orifice plate An orifice plate is placed next to the lower flanged end of the filter chamber. These flanged ends have machined-smoothed surfaces. A rubber gasket is placed between the lower'flanged end and this orifice plate. It has an opening considerably greater than that of the orifice plate. The orifice plate is a metal disc in which there is a smooth circular opening of exactly one square inch area, in its center. This orifice plate is made of corrosion-resisting material and the area of its opening is checked frequently to assure permanence of the same. Theoriflce plate is of stock to protect it against deformation in its plane surface when bolting the plate, filter pad, support and discharge pipe to the lower flanged end of the filter chamber. The orificev plate has an external circular size which equals that of the circle that is circumscribed with only a little clearance within the bolt circle of the lower flanged end of the filter chamber.

Filter pad The filter pad is a combination of four sheets of cotton fabric popularly called airplane cloth. This fabric is unbleached. The thread count per square inch is 130 x 142,warp x woof. It weighs 3 ounces per square yard. It has a thickness of 0.006 inch. In making a pad, the sheets are cut 1 each to a size of four inches on a square side and in the two respective directions of its component threads. This filter pad is placed against the lower side of the orifice plate without another inserted gasket present. In finally bolting all the parts to the lower flanged end of the filter chamber, the orifice is seated firmly over this filter pad and thereby there is exposed to the passage of the viscose sample only one square inch area of this filter pad to filter the viscose.

Support To support the filter pad with additional firmness against the orifice and further to prevent its sagging and so a distortion of its area and texture in the surface that is exposed as a filtering medium against the passage of the viscose, there is placed-under the pad two metal parts. The first is a piece of woven wire corduroy cloth, 12 x 64 I Discharge pipe This is a inch I. D. pipe connection to the opened apex of an inverted cone which extends down from a flange whose size and' bolt circle equals that of the lower flanged end of the filter chamber. This flange and the two flanged ends of the filter chamber are each of inch thickness to prevent their distortion when bolting the parts together. This discharge pipe is placed below the perforated grid support, and when the orifice, filter pad, and two supports are aligned, it is bolted to the flanged end of the filter chamber, with mechanical care used to apply equal firmness around the bolt circle.

Operation if a determination of filterability The open filter chamber, first, has been scrubbed with an oversize, round, coarse brush and rinsed clean and drained. The orifice plate,

filter pad, supports and'discharge pipe'are assembled and bolted on to the lower end of the filter chamber. The jacket water is adjusted to 20 C. and fiowed to temper the assembled filter to 20 C. The 20 C. viscose is poured carefully and slowly into the chamber and the top disc bolted on. Air pressure at 70 pounds is applied. At the first moment of discharge of filtrate, an interval clock is started. The filtrate is received into a calibrated glass receptacle, and the accumulations in cc. are read periodically. The discharged cc. are plotted continuously as abscissae against total time as ordinates. When the curve shows that the rate of discharge of the filtrate approaches zero in relation to elapsed time, then the pad is known to be plugged. An interval of several minutes will have occurred in which no filtrate is discharged. The total cc. of filtrate discharged is the measure of filterability of the viscose.

Always there is. used a new filter pad for each determination, and the orifice plate, gaskets, supports and discharge pipe are always freshly washed and dried before use. The woven corduroy cloth is blown dry with compressed air and inspected to assure that none of the mesh openings are closed.

Quality standard of filtembz'lzty I have determined that for the satisfactory manufacture of rayon from viscose, a sample of the viscose tested by the foregoing method should deliver a total of at least 500 cubic centimeters of filtrate.

The solution usually employed as a standard for measuring the viscosity of cellulose pulp is a cuprammonium cellulose solution of prescribed cellulose concentration, the viscosity being determined by measuring the time of efllux of a definite volume of such solution under standard conditions, through an orifice of standard size. The solution viscosity of fiber is hereinbefore given in absolute C. G. S. units or poises, and is. determined by measuring the Viscosity of a solution of 6 grams of fiber in a cuprammonium solution composed of 225 cc. of 28% ammonia Water containing 9 grams of so-called "copper hydrate powder. This copper hydrate powder is in reality a basic copper nitrate. The C. G. S. unit is employed because it is definite, denoting a viscosity 100 times that of water at 20 C., wherefore, a cuprammonium cellulose solution of standard composition identifying a fiber as having a solution viscosity of 10 is 1000 times as viscous as water at 20 C. The method of determining or measuring solution viscosity of cellulose fiber used herein is that described by me in much greater detail in Industrial and Engineering Chemistry, volume 23, page 136, 1931; and inasmuch as the description of my viscosity-testing method --as given in that publication affords the particular criterion or test used herein, it is to be understood that my reference to such description is intended to incorporate such description as a part hereof. Since the methods of determining alpha-, beta-, and gamma-cellulose used for the purpose of the invention hereof are also set forth in detail on pages 134 and 135 of the publication just mentioned, it is to be further understood that the de- .scription of such methods as given in that publi-' cation are intended to be made part hereof.

I claim:

.caustic soda and cellulose .less than 1; washing the fiber substantially free of such treating solution; and mixing the fiber with caustic soda solution and liquid carbon bisulphide in amount to xanthate the fiber substantially completely and to form viscose.

2. A process of producing viscose, which comprises treating bleached chemical wood pulp at about 30 to 60 C. with caustic soda solution of a strength upwards of about 6% and containing oxidant in sufiicient amount to cause a reduction of the solution viscosity of the pulp to a value below 1; continuing the treatment for a sufiicient period of time to yield a pulp product having an alpha cellulose content of about 87 to 93%; a beta cellulose content of about 6 to 12%, and a viscosity less than 1; washing the pulp product substantially free of such treating solution; and} mixing the resulting pulp product with caustic soda solution and liquid carbon bisulphide in amount to xanthate the fiber substantially completely and to form viscose.

3. A process of forming viscose in one step, which comprises commingling caustic soda solution and liquid carbon bisulphide with cellulose fiber having an alpha cellulose content of about 87 to 93%, a beta cellulose content of about 6 to 12%, and a solution viscosity less than 1. the causticity of said solution and its proportion relative to the fiber being controlled to yield; viscose of a. content of about 6% to 10% each.

4. Cellulosefiber capable of being substantial- .ly completely xanthated and dissolved to form viscose by direct admixture with caustic soda solutio'n and liquid carbon bisulphide, said fiber having the following characteristics:

Alpha cellulose content 87 to 93% Beta cellulose content 6 to 12% Viscosity less than 1 5. Cellulose fiber capable of being substantially completely xanthated and dissolved to form viscose by direct admixture with caustic soda solution and liquid carbon bisulphide, said fiber having the following characteristics:

Alpha cellulose content 87 to 93% Beta cellulose content 6 to 12% Gamma cellulose content lto 3% Viscosity less than 1 mixture having the following characteristics:

Alpha cellulose content 87 to 93% Beta cellulose content 6 to 12% Viscosity less than 1 GEQRGE A. RICHTER. 

